Selective circuits for mutiplex signaling



oct. 9, 1923. 1,469,832

B. P. HAMILTON SELECTIVE CIRCUITS FOR MULTIPLEX SIGNALING Filed Sent. 23, 1919 INVENTOR.

Patented Oct. 9, 1923.

UNITED? STATES PATENT OFFICE. i

EAxTEE P. HAMILTON, or BROOKLYN, NEW YoEx,"AssIGNOE To AMERICANv TELE- PHONE .AND TELEGEAPII COMPANY, A'CORIPOEATIONv 0E NEW YORK.

- SELECTIVE CIRCUITS FOR MUTIPLEX SIGN-ALING.

f Application filed September 23, 1919. Serial No. 325,681.

To all whom t may conceive.'

Beit known that I, BAXTER P. HAMILTON, residing at Brooklyn, in the county -of Kings and State lof New York, have in- 5 vented ,certain Improvements in Selective Circuits for Multiplex Signaling, of which the following isa specification.` v

This invention relates to multiplex signaling and more particularly to an arrangement of selective circuits for maintaining the channels of a mutiplex system electrically separate. l

In order to obtain themaximum service from a pair of wiresusedV for telephonie transmission, itis desirable `that as many signaling channels as possible be lsuperposed upon the pair ofwvires, in order to utilize as great arange ofthe frequency spectrum as possible. If the frequency spectrum` be considered to comprise all ofthe frequencies lying between zero and infinity, `it will be seen that the frequencies employed in ordinary telephonic transmission, which may be consideredto lie between 200 and 2500 cycles zo per second, occupy but a relatively small part of the total frequency spectrum, so that it is possible lto superpose channels upon the pair of wires, both above `and below the telephonie range. The upper limit of frequencies available for superposition of. channels above. the telephonie range will, of course, be far short of infinity, `because of the limitations imposedzby the transmission characteristics of the line. In the case of ordinary open wire circuits, .itis noty ordinarily practicable to transmit @frequencies higher than about 30,000 cycles. l In accordance with the present invention it is proposed to utilizethefmaximum extent of the practicable signaling range, by superposing on the pair yof wires, in addition to the ordinary telephone channel an ordinary Morse channel whose frequency is in the neighborhood of about 16 cycles, one or morealternating current telegraph channels having frequencies in the neighborhood of 100 cycles and one or more high frequency carrier channels which may be used for either telegraph or ltelephone signaling, having vfrequencies lfromlsay 4,000 or 5,000 cycles upto the highest frequency which 'can be eiciently transmitted. An additional channel for the purpose of transmitting ringing signals can .also be superposed on the same pair of wires, this channel..having assigned to it a frequency Within thevoice range. It is possible to do this by reason of the fact that ringing and talkingdo not take place simultaneously.

In order to provide proper frequency separation between the several channels superposed upon the same conductor, an arrangement of filters is provided in accordance with this invention, said arrangement comprising a number of separate broad band filters, certain of which may be upper limiting or low pass filters, others of which may be lower limiting or-high pass filters, and still other of which may be filters transmitting a band of frequencies whose upper and lower limits lie below infinity and above zero respectively, in the total frequency spectrum. -These filter units are soassociated with the common transmission path and the several superposed branches and are so designed as to render the several branches mutually exclusive as regards the frequencies of all other branches.

The invention may now be readily understood from the following description when read in connection with the accompanying drawing, Figures 1, 2 and 3 of which constitute circuit diagrams of three different embodiments of the invention.

Referring to Fig. l, ML designates a telephone transmission line upon which a plurality of signaling channels maybe superposed in the manner about to be described. L1, L2, L3, L4 and L5 designate a plurality of branches whereby signaling channels may be established over the line. The line L.L may beA used for metallic circuit Morse signaling. The average frequency used for signaling of this character may be in the neighborhood of 16 cycles. The line L2 may be utilized for transmitting telegraph signals by low frequency alternating currents. The line L3 may be utilized for impressing ordinary telephonie frequencies upon the line. These frequencies may be assumed to lie between 200 and 2500 cycles. L may be used for impressing one or more relatively. high frequency carrier signaling channels upon the line. l These carrier channels may, of course, be used 'for the transmission of either telephone or telegraph Signals.

In order to provide for frequency separa- The line preassigned` limits. They filters may be in general of three types, depending upon the location of the bandtransmitted by the filter with-respect to the entire frequency spectrum. The first type is the so-called low pass or upperl limiting vfilter' which transmits a band of frequencies lying between zero andy any upper limiting frequency greater than zero, but less-than infinity. The 'sec- `ond type is the so-called high pass or lower limiting lte'r" which transmits a band of frequencies lying between infinity and a lower limit greater than zero. The third type transmits a band lying between a', lower limit greater than zero and an upper limit less than' infinity. The low pass and high pass filtersv are sometimes known as ultra and infra filters respectively.

In order to carry out the principles of this invention' an ultra or a low pass filter LF1 is inserted in the line L,. Each section of this filter, it will be noted, consists of series inductances, `and shunt capacity, theseelements being so proportioned as to make the upper limiting frequency transmitted by the filter about 30 cycles. Since this frequency is lower than the lowest frequency transmittedover any of the other branches, L2, L3, etc., it serves to prevent frequencies from the other channels from producing interference on the line L1, while affording a path of low impedance to Morse frequencies transmitted over the yline L1. In order to prevent these Morse frequencies from reacting in anyfof the other circuits an infra or high pass filter H F2 is inserted in the line ML betweenits junction with the line Ll and its junction with the next higher line L2'. The filter HF 2, it will be noted, comprises a plurality of sections, each section including series capacity and shunt inductances, these elements being so proportioned that the filter will freely transmit frequencies from 80 cycles up to infinity, but will substantially suppress frequencies below 80 cycles. Since the frequencies from the lines L2, L3, etc., are all above 80 cycles, these frequencies may be transmitted through the filter I -IF2 to the line ML, but since the frequency transmitted over the line L1 is below the lower limiting frequency of the filter I-IF2, the 'Morse frequencies cannot react on the lines L2, L3, etc.

An ultra or low pass filter LF, is included in the line L2, said filter` being so designed as toitransmitfrequencies vfrom zero to 120 cycles, thus affording a path ofv lowimpedance for an alternating telegraph transmission having a frequency of about y100 cycles. v This filter serves to prevent higher frequencies from the lines or branches to the left of the line L2 from reacting onk the latter line. In order to prevent the frequencies incoming from the lineL2 from reacting on the higher frequency lines to the left thereof and at the same time readily permit frequencies from such otherlines to be transmitted to the main line ML, a high pass filter IIFsis connected in the main line between its junc-` tion with the line L2'a`nd its junction with the line L3. This filteris designed to transmit frequencies from 200 cycles up.

HF4 is inserted in the connection between i the mainline and the branch L4. This filter, in the case illustrated, has a lower cut-ofi' of 4,000 cycles, thereby freely transmitting carrier frequencies from 5,000 `cycles up to say 25,000 or 30,000 cycles, these frequencies being the highest frequencies it is practicable to transmit over an ordinary telephone line as now constructed, Obviously if the transmission line "is adapted to transmit `higher frequencies, additional carrier channels of higher frequency may be employed.

In addition to the channels hereinbefore described, it is usually necessary in connection with the telephone channel L3, to pro-A vide nieansfor ringing over the main line ML. For this purpose an additional branch L5 is providedV between the filter HF 3 'and its junction with the line L3. This channel may be used for transmitting ringing frequencies` in the neighborhood of 300 cycles. While this frequency lies within the range of ordinary telephonie transmission incoming from the line L3, it is possible to use a frequency in this range, by reason of the fact that ringing current will not, in practice, be transmitted to the line while talking, and consequently the channel L,5 will be disconnected `during telephonie transmission. This channel is provided with a filter BF, which, it will be observed, is composed of sections having i'nductance and* capacity in series and parallel induct-ance and capacity in shunt with the line.l .A filter of this type falls within the third Classification hereinquencies quite close together upon either side of 300 cycles. With a filter thus constructed, it is obvious that frequencies from channels transmitting frequencies any considerable extent above or below 300 cycles cannot react upon the channel L5.

A modied arrangement is shown in Fig. 2. In this form of the invention the main line ML has Morse legs L,L and L1 associated therewith, so that each side of the line may be utilized for the transmission of Morse currents. Branches L2, L3 and L4 are also provided for superposing alternating current telegraph signals, ordinary telephone frequencies and high frequency carrier channels upon the main line ML. The Morse legs L,L and L1 are provided with low pass filters LF,L and F1', respectively, for freely transmitting Morse frequencies while suppressing the high frequencies which are transmitted over the branches L2, L3 and L4. The branch L2, which is to be used for the transmission of frequencies in the neighborhood of 100 cycles, is provided with a band filter BF2 whose upper and lower limiting frequencies lie closely adjacent on either side of 100 cycles. This filter may be of the general construction of the filter BF of Fig. 1, that is, each section should comprise series inductance land capacity, with parallel inductance and capacity in shunt with the line. This filter will, of course, prevent Morse signals as well as higher telephone and carrier frequencies from reacting on the circuit L2.

The telephone line or branch L3 is also provided with a band filter BF3 of construction similar to the band filter BF of Fig. 1. The inductance and capacity elements of this filter are so designed as to make its upper and lower cut-off frequencies about 2500 and 200 cycles respectively. This will allow free transmission of telephonie frequencies to the main line ML and will prevent the reaction of the lower telegraph frequencies or the higher carrier frequencies from the other` branches. The branch L4, which is intended for the transmission of carrier currents to be utilized for either telephone or telegraph signaling, may be equipped with a filter and since there is no necessity for providing against the reaction of higher frequencies in this circuit, it is sufficient to include in the circuit merely a high pass filter HF4 having a lower cut-off in the neighborhood of 4,000 cycles. This will prevent the lower frequencies from reacting on the branch L and at the same time will readily permit the passage of the high carrier'frequencies from this branch to the main line ML.

It will be noted that'in accordance with this arrangement of the invention the use of high pass filters between the junction points of the various branches with the main line, is avoidable, thus reducing the number of filters necessary, as compared with Fig. l. However, it necessitates the use of a number of the more complex filters of the third type hereinbefore referred to, which have their upper and lower cut-offs below infinity and above zero respectively. Filters of this type are, of course, more expensive than filters employing simple inductances and capacities in their shunt and series branches.

Fig. 3 shows substantially the same arrangement as that of Fig. 2, except that an additional channel is provided for ringing or signaling in connection with the establishment of a telephone connection. The several filters in the branches L2, L3 and L,E areA simply indicated schematically and it will be understood that they are constructed in the same manner as the corresponding filters BF2, BF3 rand HF4 of Fig. 2. In connection with Morse channel L1, the low pass'filter LFl, indicated schematically, will be understood to be similar to the filter LF1 of Fig. l, although if grounded Morse legs such as illustrated in Fig. 2 are to be used, it will be understood that separate low pass filters will be divided in each leg, as indicated in Fig. 2.

In order to apply the ringing signal to the telephone line, a branch connection L5 is bridged across the line L3 and the ringing signal is applied to this bridge through a transformer and tuned circuit arrangement TF. Band lters BF and BF are included on each side of the transformer TF, these band filters being of the same type as the filter BF of Fig. l. The function of these` filters is to prevent the reaction .of the lower telegraph frequencies and the higher carrier frequencies upon the branch L5. Two filters, BF and BF are provided, simply for purposes of balance. It will, of course, be understood that since ringing signals are not transmitted at the same time that talking is taking place, the branch L5 may be disconnected when the line L3 is in talking condition, thereby preventing voice frequencies in the neighborhood of 300 cycles from being diverted into the shunt path through L5.

It will be obvious, of course, that the various frequencies hereinbefore referred to and indicated upon the drawing, are merely for purposes of illustration and that in practice other frequencies than those indicated may be used, the essential requirement being merely that the frequencies assigned to the various channels and to the correspondline, a plurality of branches from said line for transmitting a` plurality of signaling frequencies superposed upon said line, said branches being vconnected across the terminals of said line in parallel, in the order of the frequencies assigned to them, so that the branch transmitting the highest frequency will lbe farthest from the line, a low pass filter in each branch, adapted to transmit frequencies between zero and an upper limit at least as great as the maximum frequency assigned to the branch and less than the lowest frequency to be transmitted over the next highest branch, and high pass filters included in the transmission line between each pair of branches, each high pass filter being arranged to transmit a band of frequencies between infinity and a lower limit which is greater than the frequency assigned to the lower branch and less than the lowest frequency to be transmitted at the higher of the two branches.

2. In a signaling system, a transmission line, a plurality of branches associated therewith, one of said branches being adapted for the transmission of Morse signals, another branch being adapted for the transmission of low frequency alternating current telegraph signals, a third branch being designed for the transmission ofv voice frequencies and a fourth branch being designed for the transmission of carrier frequencies higher than voice frequencies, upper limiting filters in certain of said branches, the upper limit of the filter in the Morse branch being greater than the highest frequency used in Morse signaling and less than the low frequency alternating telegraph frequency, the upper limit of the filter in the alternating current telegraph branch being higher than the frequency assigned to said branch and lower than the lowest Morse frequency, the upper limit of the filter in the telephone branch being at least as high as the highest voice frequency to be transmitted and lower than the lowest carrier frequency of the fourth branch, and high pass filters between each pair of branches,the lower limit of each high pass filter lying between the highest frequency assigned to the branch immediately below it and the lowest frequency assigned to the branch immediately above.

3. In a selective system, a plurality of pairs of. filters, each pair of filters comprising a high-pass filter and a low-pass filter, a circuit for impressing currents of different frequencies on said system, a plurality of branch circuits for receiving selected ranges of said impressed frequencies, the frequencies transmitted to each branch being the frequencies that are transmitted selectively by` the joint action of the filters of at least one of said pairs of filters, and transmission also taking y *place successively through certainfilters of different pairs.

4. In a selective system, a plurality of pairs of filters, each pair of filters comprising a high-pass filter and a low-pass. filter, acircuit for4 impressingcurrents of different frequencies` on said system, branch circuits for receiving selected ranges of the impressed frequencies, the frequencies transmitted to each branch circuit being the frequencies that are transmitted selectively by the filters of at least one of said pairs of filters, and the frequencies transmitted to certain of said branches being the frequencies that are transmitted by one pair of said filters and at least another of said filters in common.

5. A selective system comprising a line having a plurality of filters in series in said line and a succession of branched circuits for said line, one branch being connected to` the output terminals of each of said filters, each said filter of the series freely transmit-- ting currents of all the frequencies common to all the branched circuits: except those preceding it in t-he series, and a. filter kfor each branched circuit freely transmitting for said branched circuit currents of frequencies which are transmitted in common by all said filters preceding it in the series, said filters substantially extinguishing currents of the frequencies which they do not freely transmit.

6. A pair of `filters connected in series having a transmission range of frequencies coinmon to both filters, one of said filters substantially extinguishing currents of all frequencies lower than said range of frequencies, the other of said filters substantially extinguishing currents of all frequencies higher than said range of frequencies, another pair of filters similarly associated with each other, one of which is connected in series with the corresponding filter of the first pair and a branch circuit connected t each of the other filters.

7; Pairs of filters, the filters of each pair being connected in series and having a transmission range of frequencies common to both filters, one of said filters ofy each pair substantially extinguishing currents of all frequencies lower than said range of frequencies, the other of said filters of said pair substantially extinguishing currents of all frequencies higher than said range' of frequencies, the corresponding filters of the successive pairs being connected in series and a branch circuit connected to each of the other filters, each of said corresponding filters that are connected in series extinguishing a portion of the currents that are freely transmitted by the preceding lter of the series.

S. A series of filters having successively different sharply defined cutoff frequencies and each substantially extinguishing currents of all frequencies lying to the same side of its respective cutoff frequency but freely transmitting currents cf all frequencies lying to the opposite side of said cutoff frequency, a branch circuit connected to the output terminals of each of said series filters containing a filter having a sharp-ly defined cutoff frequency and substantially extinguishing currents of all frequencies lying to one side of said cutoff frequency but freely transmitting a range of the frequencies that are transmitted by the filter to which it is connected.

9. The combination of filters each of which freely transmits currents of all frequencies higher than a predetermined cutoff frequency and substantially extinguishes currents of all frequencies lower than said cutoff frequency, other filters each of which freely transmits currents of all frequencies lower than a predetermined cutoff frequency and substantially extinguishes currents of all frequencies higher than its said cutoff frequency, said filters being associated in pairs consisting of one filter of each kind in series,.having a transmission frequency range in common, all the filters of one kind being connected in series and a branch circuit connected to each filterof the other kind.

10. The combination of lters each of which freely transmits currents of all frequencies higher than a predetermined cutoff frequency and substantially extinguishes currents of all Jfrequencies lower than said cutoff frequency, other filters each of which freely transmits currents of all frequencies lower than a predetermined cutoE frequency and substantially extinguishes currents of all frequencies higher than its said cuto frequency, said filters being associated by pairs consisting of one filter of each kind in series, having a transmisison frequency range in common, all the filters of one kind being connected in series and a branch circuit connected to each filter of the other kind, the cutoff frequency of each filter of said one kind in series being within the transmission range of all of the filters which precede it in the series.

11. A selective circuit comprising filters of two kinds, each filter having a sharply defined cutoff' frequency, certain of said filters comprising reactance elements which substantially extinguish currents of all frequen- Y cies greater than their respective cutoff' frequencies, while freely transmitting currents of all other frequencies, and others of said filters comprising reactance elements which substantially extinguish currents of all frequencies lower than their respective cutoff frequency while freely transmitting currents of all other frequencies, a plurality of filters of one kind connected in series relation, filters of the other kind connected to the output terminals of the respective filters of said one kind, the transmission range of each filter of said other kind overlapping that of the filter to which it is connected.

12. A selective circuit comprising filters of two kinds, each filter having a sharply defined cutoff frequency, certain of said filters comprising reactance elements which substantially extinguish currents of all frequencies greater than their respective cuto frequencies, while freely transmitting currents of all other frequencies, and others of said filters comprising reactance elements which substantially extinguish currents of all frequencies lower than their respective cutoff frequency while freely transmitting currents of all other frequencies, a plurality of filters of one kind connected in series relation, lters of the other kind connected to the output terminals of the respective lters of said one kind, the transmission range ofA each filter of said other kind overlapping that of the filter to which it is connected, and the transmission range of each filter of one kind being included in the transmission range of each of the filters which precede it in the series.

13. In a system of filters comprising filters which transmit substantially only frequencies lo-wer than an upper limiting frequency and other filters which transmit substantially only frequencies higher than a lower limiting frequency, a plurality of filters of one of said kinds connected in series and a branch circuit connected between each two of said filters, each of said branch circuits containing a filter ofthe other of said kinds having a transmission range in common with those filters that are in series and to one side of the respective branch circuit.

14. A plurality of pairs of filters one filter of each pair being a low-pass filter and the other a high-pass filter, a circuit common to all of said filters for conveying currents of different frequencies successively through certain filters of different pairs, and a branch circuit individual to each of said pairs of filters for conveying currents of only the frequencies that are transmitted by the filters of said pair.

In testimony whereof, I have signed my name to this specification this 20th day of September 1919.

BAXTER P. HAMILTON. 

